add: basic imgui context

.gitmodules

README.md

src/app/rmrf.cpp

src/app/rmrf.ldflags

src/lib/imgui

src/ui-imgui/graphics_context_gl3.cpp

src/ui-imgui/graphics_context_gl3.h

src/ui-imgui/imgui_theme.cpp

src/ui-imgui/imgui_theme.h

src/ui-imgui/ui_exception.cpp

src/ui-imgui/ui_exception.hpp